An Ab Initio Molecular Orbital Study on the Conformations of Bicyclo[3.1.0]hexane Derivatives

Ab initio calculations have been carried out for 2,4-dithia- (5), 2,4-dioxa- (6), 6-oxa- (8), 6-thiabicyclo[3.1.0]hexanes (9), bicyclo[3.1.0]hexane (7), and some of the corresponding cyclohexane derivatives 10–12 using 3-21G, 3-21 G(*) and 6-13G* basis sets. Relative total energies and geometries have been obtained for both boat and chair forms of these compounds. The calculated geometries are in good agreement with observed ones for 7–9. The boat form is more stable than the chair form in the bicyclic systems 7–9, while both the forms have almost equal stability for 5 and the chair form of 6 is slightly more stable than the boat form. The relative stability of the boat form in the bicyclic systems compared with that in the monocyclic cyclohexane derivatives can be explained in terms of the stabilizing orbital interaction. Inversion barriers between the boat and chair forms for 5–7 have also been estimated and are briefly discussed. A similar calculation for bicyclo[3.2.0]hept-6-ene also shows that the boat form is much more stable than the chair form contrary to the conclusion derived from the analysis of experimental results.